Without limiting the scope of the invention, its background is described in connection with logarithmic amplifiers used in a power detector.
In a wireless communication system, for example, a Global System for Mobile (GSM) system using a Time Division Multiple Access (TDMA) signaling format that includes a framed structure comprising eight time slots, a mobile station communicates with a base station by transmitting and receiving information in one or more of the time slots that comprise a channel. Each channel is assigned to a different user, with mobile-to-base transmission (uplink) on one frequency band and base-to-mobile (downlink) on a second frequency band.
In order to preserve the integrity of the transmitted and received information and to reduce adjacent channel interference, the system operates according to a standardized format that defines the requirements of transmission and reception. A system transmitting and receiving information often produces unwanted interference. This unwanted interference affects the integrity of the transmitted and received information. For example, a power control loop uses negative feedback to adjust the operating point of a power amplifier so that the power amplifier operates in a specified range. However, unwanted interference parameters inherent in the operation of the power control loop may cause an inaccurate representation of the information to be controlled in the feedback loop resulting in inaccurate adjustment of the power amplifier's operating point.
The feedback control loop controls the operation of the power amplifier by using an RF linear detector to sample the output signal and compare the output signal with a reference signal, where the reference signal is proportional to the required output. The RF linear detector output is used as an error signal to adjust the power amplifier's operating point to correct any unwanted deviations detected at the output. Unwanted interference parameters of the RF linear detector could affect the signals in the loop and may result in an incorrect adjustment of the power amplifier.
Reference is now made to FIG. 1, wherein a prior art logarithmic amplifier used in RF linear detectors is illustrated and denoted generally as 10. Logarithmic amplifier 10 includes an operational amplifier 12 and a diode 14 that operates in the small signal region. A small signal input I.sub.1 is connected to the inverting input of operational amplifier 12, and the non-inverting input is coupled to ground through resistor R.sub.4. Bias voltage V.sub.s is coupled to the inverting input and the anode of diode 14, through a current limiting resistor R.sub.b, and produces a bias current I.sub.b that biases diode 14. The output of operational amplifier 12 is coupled to the cathode of diode 14 through resistor R.sub.0. Output V.sub.o of logarithmic amplifier 10 is taken from the output of operational amplifier 12.
Ideally, output V.sub.o should be a true representation of the logarithmic value of I.sub.1 ; however, there are parameters of the logarithmic amplifier 10 which produce variations in output V.sub.o. A saturation current I.sub.s (T), in diode 14, is a function of temperature and causes variations of the output V.sub.o when operating at different temperatures (T). Bias current I.sub.b, generated by V.sub.s, also is an unwanted parameter at output V.sub.o that affects the linearity by introducing an additional constant voltage at output V.sub.o. The effects of these interference parameters on the output V.sub.o can be seen from equation 1 below, which represents the output V.sub.o of the logarithmic amplifier 10 of FIG. 1. ##EQU1##
As may be seen from Equation 1, an improved apparatus to effectively remove interference parameters from the output of a logarithmic amplifier could improve the accuracy and performance of the logarithmic amplifier.